1. Field of the Invention
The present invention relates to storage stable, aqueous polyurethane-polyacrylate hybrid dispersions, which are self crosslinkable at room temperature, to their use as binders in storage stable, one-component coating compositions and to coated substrates prepared from these coating compositions.
2. Description of the Prior Art
It is known that aqueous polyurethane dispersions are suitable for the production of adhesives and coatings. In addition to a good chemical resistance, films prepared from polyurethane dispersions are distinguished, above all, by abrasion resistance, toughness and good adhesion to many substrates, e.g., wood, metal and plastics.
Polyurethanes are usually prepared by the isocyanate polyaddition process. Various processes for the preparation of these binders have been summarized, e.g., by D. Dieterich in Prog. Org. Coatings 9, 281 (1981).
It is known to carry out a free radical polymerization of vinyl monomers in the presence of aqueous polyurethane dispersions to modify the properties of the dispersions in a suitable manner as disclosed, e.g., in DE-A 1,953,348 and in EP-A 167,188, 189,945 and 308,115.
It is also known to incorporate terminal or lateral .alpha.,.beta.-olefinically unsaturated double bonds into a polyurethane initially as disclosed, e.g., in EP-A 098,752, 522,419, 522,420 and 649,865). These polyurethane macromonomers are said to offer the advantage of better bonding to the polyacrylate, which is prepared in the following step by free radical copolymerization. However, it is disclosed in DE-A 1,953,348 that a more or less pronounced grafting also occurs during free radical polymerization in the presence of a polyurethane that does not contain unsaturated groups. It is also disclosed that the high crosslinking density resulting from the use of unsaturated polyurethanes can sometimes be a disadvantage.
To further improve the resistance properties of polyurethane or polyurethane-polyacrylate hybrid dispersions, various possibilities have been proposed for chemically crosslinking the films prepared from these binders. Compared with two-component coating or stoving compositions, one component compositions, which are storage stable and crosslink chemically at room temperature without addition of a crosslinking component, offer advantages because they are easier to use.
It is known to provide polyurethane dispersions with groups that undergo oxidation drying. Films of such dispersions crosslink under the influence of atmospheric oxygen ("Advances in Urethane Science and Technology", vol. 10, p. 121-162 (1987)).
EP-A 332,326 describes mixing polyurethane dispersions containing carbonyl groups with polyacrylate dispersions containing hydrazine functional groups, or polyacrylate dispersions containing carbonyl groups with polyurethane dispersions containing hydrazine functional groups. After film formation, crosslinking takes place by reaction of the carbonyl groups with the hydrazine groups.
EP-A 649,865 discloses a process for the preparation of self-crosslinking polyurethane-polyacrylate hybrid dispersions based on polyurethane macromonomers grafted with acrylate monomers containing carbonyl groups. Polyhydrazides are added to the dispersions as crosslinking agents.
However, self-crosslinking dispersions with crosslinking agents containing hydrazine groups have the disadvantage that they can release small amounts of hydrazine.
Another possibility that has been proposed is the crosslinking of carbonyl groups with polyamines to form enamine or azomethine groups. It has been described that polyurethane-polyacrylate hybrid dispersions, which have carbonyl functional groups and contain the carbonyl groups in the polyurethane part, and polyfunctional amines form a storage stable, self-crosslinking, one-component composition. Crosslinking is controlled by evaporation of the neutralizing amine (R. Tennebroek, J. Geurts, A. Overbeek, A. Harmsen, 4th Nurnberg Congress "Creative Advances in Coatings Technology", paper 17, (1997)).
The preceding compositions do not achieve the known high property level of two-component polyurethane coatings. Accordingly, it is an object of the present invention to improve the crosslinking of one-component compositions such that the properties of the resulting films possess improved properties.
Surprisingly, this object of obtaining improved properties may be achieved with the self-crosslinking polyurethane hybrid dispersions according to the present invention, which contain acetoacetoxy units incorporated in the polyacrylate portion.